threshold and indicates the least amount of exposure
many cases there will not be a well-defined straight
line. This section of the curve has a constant slope,
and in addition, the gradient of the slope here is
is an equal increase in density for an equal increase in
exposure. This is the most important part of the
curve. Subject exposures that fall on the straight line
produce constant density differences.
This section of the curve is called the shoulder. Like
the toe area, equal changes in exposure do not
produce equal differences in density. Tones of the
that is, reproduced with densities too heavy for
printing or maximum detail. For normal exposures,
bright highlight tones of the subject tend to be
reproduced in the shoulder section of the curve.
photographed, there is a single exposure,
corresponding to a single point somewhere on the
log-H axis. When the light on the surface of the gray
card is increased and another photograph is made
(maintaining the same camera settings), the exposure
and the log H also increase. This causes a shift to the
right on the log-H axis.
again should plot on the characteristic curve.
tonal differences in the subject, the lighting, and the
camera settings determine the film exposure that
produces varying amounts of densities. These density
differences must be related to the log-H differences
negative must be considered in their relationship with
the tones of the subject.
corresponding to the subject tones in the correct
section of the characteristic curve. This is done by
adjusting the camera settings correctly, providing that
the range of tones in the subject is not too great.
differences and log-H differences. In other words, it
is simply the range of exposure covered by the
straight-line section of the characteristic curve (fig.
2-6). The latitude of an emulsion, therefore,
determines the brightness range of the subject that can
be reproduced proportionally. Latitude may be
expressed either as the difference in log-H values
between the two extreme points of the straight line
or as the exposure ratio between these same two
panchromatic film, and from 1:20 or less for process
film. For any given emulsion, the emulsion latitude
varies according to the contrast. The emulsion
latitude decreases as the contrast increases.
straight-line section of the curve. For ground pictorial
film the useful exposure range of a sensitized material
is greater than the emulsion latitude, since portions of
the toe and shoulder regions of the curve are used
without sacrificing print quality.
gross fog of the film. This point is referred to as
minimum useful density. The upper limit is generally
located at 90 percent of maximum density on the
useful density. In practice, many photographers use a
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